it has nothing to do with the vent on/off switch. probably just a coincidence that the TC stopped shuddering for the time being.
usually acts up once the tranny is hot.
if it is the TC, you can continue to drive it (probably), but after a while it gets annoying and it's time for a new or re-built tranny.
typically $2K - $2500. is it worth it? depends on the condition of everything else in your V8 SHO. a decision that only you can make.
or, you can try to re-sell it to a sucker (you know...there's one born every minute!).
when i purchased my Gen3 with 76K miles, i had a shuddering tranny.
the car was in such flawless condition that i decided to do a custom re-build. it's better than new (or an OEM Ford tranny).